Because the gi can be used as a weapon, and you cant learn that without wearing one in practise. It may not be used strictly in MMA (yet?!?) but I'm sure it has cross over ideas, it might expand the mind of the fighter to position or transition himself differently, it might help him progress up the belting ranks in that particular combat sport.

These guys are not mere MMA Fighters, some are specialists in particular disciplines aswell, some compete in grappling tournements, or in pure bjj challenges outside of the UFC (thats not a breach of their contracts in certain circumstances)

I would never dissaude or think negatively of someone who would train with as well as without.